1. Field of the Invention
The present invention relates to a solid polyurethane powder coating composition containing an uretdione group that hardens at low stoving temperatures, a process to manufacture such composition and the use of this process to manufacture plastics, in particular powder coatings, which cross link to form high gloss or matt, light and weather stable coating films.
2. Discussion of the Background
Externally or internally blocked polyisocyanates which are solid at room temperature are valuable cross linkers for thermally cross-linkable polyurethane (PUR) powder coating compositions.
For example, DE-OS 27 35 497 describes PUR powder coatings with excellent weathering and thermal stability. The cross linkers whose manufacture is described in DE-OS 27 12 931 consist of isophorondiisocyanate containing ε-caprolactam-blocked isocyanurate groups. Polyisocyanates containing urethane, biuret, and urea groups whose isocyanate groups are also blocked are also known.
The disadvantage of these externally blocked systems is the cleaving-off of the blocking agent during the thermal cross-linking reaction. As the blocking agent can thus emit into the environment, for ecological and occupational hygiene reasons, special precautions must be taken to cleanse the waste gas and/or to recover the blocking agent. In addition, the cross-linkers exhibit a low level of reactivity. Hardening temperatures of above 170° C. are required.
DE-OS 30 30 539 and DE-OS 30 30 572 describe processes for the manufacture of polyaddition compounds containing uretdione groups whose terminal isocyanate groups are irreversibly blocked with monoalcohols or monoamines. One particular disadvantage are the chain-breaking components of the cross-linkers which lead to low network densities in the PUR powder coatings and hence to poor solvent resistance.
Hydroxyl group-terminated polyaddition compounds containing uretdione groups are the subject of EP 669 353. Because of their functionality of two, they have an improved resistance to solvents. The powder coating compositions based on these polyisocyanates containing uretdione groups have one thing in common, namely that they do not emit volatile compounds during the hardening reaction. However, the minimum stoving temperature of 180° C. is high.
The use of amidines as catalysts in PUR powder coating compositions is described in EP 803 524. Although these catalysts lower the curing temperature, they result in considerable discoloration, which is generally undesirable in the coating area. The cause of this discoloration is probably the reactive nitrogen atoms in the amidines. These can react with atmospheric oxygen to form N-oxides that are responsible for the discoloration.
EP 803 524 mentions other catalysts that have been used for this purpose in the past without exhibiting any special effects on the curing temperature. These include organometallic catalysts known from polyurethane chemistry, such as dibutyl tin dilaurate (DBTL), and tertiary amines such as 1,4-diazabicylco[2.2.2]octane (DABCO).
WO 00/34355 lays claim to catalysts based on metal acetylacetonates, e.g. zinc acetylacetonate. Catalysts such as this are in fact able to lower the curing temperature of polyurethane powder coating compositions containing uretdione groups, but the main reaction products are allophanates (M. Gedan-Smolka, F. Lehmann, D. Lehmann “New catalysts for the low temperature curing of uretdione powder coatings”, International Waterborne, High solids and Powder Coatings Symposium, New Orleans, 21-23. 2. 2001). Allophanates are the conversion products from one mol alcohol and two mols isocyanate, while in conventional urethane chemistry one mol alcohol reacts with one mol isocyanate. Hence, technically and economically valuable isocyanate groups are destroyed due to the undesired formation of allophanates.